[@Property] Enable by default

https://bugs.webkit.org/show_bug.cgi?id=249774
rdar://103637580

Reviewed by Tim Nguyen.

CSS Properties and Values API
https://drafts.css-houdini.org/css-properties-values-api/

* Source/WTF/Scripts/Preferences/WebPreferencesExperimental.yaml:

Flip the switch.

Canonical link: https://commits.webkit.org/258246@main

Source/WTF/Scripts/Preferences/WebPreferencesExperimental.yaml

@@ -328,11 +328,11 @@ CSSCustomPropertiesAndValuesEnabled:
   humanReadableDescription: "Enable CSS Custom Properties and Values API"
   defaultValue:
     WebKitLegacy:
-      default: false
+      default: true
     WebKit:
-      default: false
+      default: true
     WebCore:
-      default: false
+      default: true
 
 CSSGradientInterpolationColorSpacesEnabled:
   type: bool